In recent years, clean liquid fuels, which have low sulfur and aromatic hydrocarbon content and are environmentally friendly, are required from an environmental viewpoint. From such a viewpoint, a method of using the Fischer-Tropsch synthesis reaction (hereinafter referred to as “FT synthesis reaction”) in which carbon monoxide gas (CO) and hydrogen gas (H2) are source gases has been studied as a technique capable of producing fuel-oil base materials which do not contain sulfur or aromatic hydrocarbon components and are rich in aliphatic hydrocarbons, particularly, kerosene and gas oil base materials. In this method, natural gas is reformed to produce synthesis gas (mixed gas having CO and H2 as main components), hydrocarbons having a broad carbon number distribution are synthesized by the FT synthesis reaction from this synthesis gas, and the obtained hydrocarbons are hydrotreated and are fractionally distilled to produce a liquid-fuel base material. This method is referred to as a GTL (Gas-to-Liquid) technique (for example, refer to Patent Document 1).
Additionally, as a method of producing hydrocarbons by the FT synthesis reaction, a method is known using a bubble column slurry bed reactor in which blows synthesis gas into a slurry having solid catalyst particles suspended in liquid hydrocarbons (hereinafter simply referred to as “slurry”) to perform the FT synthesis reaction (for example, refer to Patent Document 2).
In the method using this bubble column slurry bed reactor, a gaseous phase portion is formed at an upper portion of the slurry within this reactor, and the synthesis gas (unreacted synthesis gas), which is unreacted while passing through the inside of the slurry, and light hydrocarbons, which are produced by the FT synthesis reaction and are gases under conditions within the reactor, are discharged from a conduit connected to an upper portion of the reactor.
In such a bubble column slurry bed reactor, typically, in order to separate and recover liquid light oil from the light hydrocarbons discharged from the conduit connected to the upper portion of the reactor, the conduit is connected to a vapor-liquid separator, a gas discharge component from the upper portion of the reactor is cooled by a cooler of the vapor-liquid separator, and condensed light hydrocarbons are separated from a gas component by a vapor-liquid separation tank. Then, the gas component including the separated unreacted synthesis gas is recycled to the reactor, and the separated liquid component (light oil) is provided to a distillation process at a subsequent stage together with heavy oil to be described below.
Here, although the heavy oil produced by the FT synthesis reaction is basically discharged as a liquid from a slurry bed of the reactor, the heavy oil has slight vapor pressure under conditions within the reactor. Therefore, a portion of the heavy oil is present as gas in the gaseous phase portion and is discharged from the conduit as a portion of the aforementioned gas discharge component. Moreover, it is also considered that liquid heavy oil is entrained in the gas discharged as droplets and is included in the discharge component.
Incidentally, in the aforementioned bubble column slurry bed reactor, for example, in a start-up stage where supply of the synthesis gas (source gas) is started from an operation stop state or when the FT synthesis reaction needs to be temporarily stopped due to a particular situation, the operation of stopping the supply of the source gas, circulating nitrogen gas into a reaction system, and maintaining a flow of the slurry, though a reaction stops, may be performed. Additionally, for example, in a midway stage shifting from such an operation to a normal operation, the operation of supplying the source gas, but setting the reaction temperature to a temperature lower than that in the normal operation thereby keeping the FT synthesis reaction from proceeding substantially or performing operation in the reaction conversion rate of carbon monoxide gas markedly lower than that of the normal operation may also be performed.
In such an unsteady operation, a tendency in which cooling efficiency declines and the temperature of an outlet of a cooler rises may be seen in the cooler for cooling the gas discharge component discharged from the gaseous phase portion of the bubble column slurry bed reactor to liquefy a portion of the gas discharge component. This is because the heavy oil, which vaporizes from the liquid hydrocarbons constituting the slurry contained in the bubble column slurry bed reactor and becomes a portion of the gas discharge component, is cooled in the cooler and is deposited on and adheres to a line of the cooler as a solid (wax). In addition, there is also a possibility that problems may occur such that continuation of the operation of an FT synthesis unit becomes difficult due to a rise in the outlet temperature of the cooler or the line of the cooler is blocked in extreme cases.
As a countermeasure against problems resulting from the adhesion of the wax in such a cooler, for example, a method of melting and removing the adhered wax with steam in a stage where the cooling efficiency of the cooler has declined to a predetermined level is also considered. However, since the operation of the vapor-liquid separator is temporarily stopped in that case, the operation rate of the FT synthesis unit will decline. Additionally, although a configuration in which a plurality of vapor-liquid separators are installed in parallel so as not to stop the operation of the vapor-liquid separators is also considered, enlargement of a facility and an increase in facility costs will be caused in that case.
The present applicant has previously suggested a hydrocarbon-producing apparatus and a hydrocarbon-producing method that prevents wax from being adhered to a cooler of a vapor-liquid separator under such a background (refer to Patent Document 3).
In the hydrocarbon-producing apparatus and hydrocarbon-producing method, a supply line for a light component of light oil, which supplies the light component of the light oil within a downstream line to an upstream line, is provided between the downstream line that is located on the downstream side of a vapor-liquid separation unit at the last stage of a vapor-liquid separator and that allows the light component of the light oil whose cloud point is lower than the outlet temperature of a cooler in the vapor-liquid separation unit at the last stage to flow therethrough, and the upstream line that is located on the upstream side of the vapor-liquid separation unit at the last stage of the vapor-liquid separator, whereby wax is prevented from being adhered to the cooler of the vapor-liquid separation unit at the last stage of the vapor-liquid separator.